1. Field of the Invention
The present invention relates to a transmission diffraction element that transmits and diffracts incident light.
2. Description of the Related Art
A device or module, such as a tunable laser, a wavelength selective switch, and an optical spectrum analyzer, may separate (or isolate) light including a plurality of wavelengths, and acquire information such as intensity, phase and the like of the light included for each wavelength. One method of separating the light may use a prism or a diffraction element. The diffraction element in particular may make a grating period short in order to make a diffraction angle difference per unit wavelength increment large.
When using the diffraction element, a plurality of diffracted light, namely, ±1st-order, ±2nd-order, ±3rd-order, . . . diffracted light, are generated. Hence, in order to compensate for the low utilization efficiency of light, the grating period of the diffraction element may be set approximately equal to or less than the wavelength of the incident light, and the incident light may be input obliquely to the direction of a normal to the diffraction element. In this case, the utilization efficiency of light may be improved by generating only the +1st-order (or −1st-order) diffracted light, as the diffracted light.
In addition, in optical communications and optical measurements, the polarization state is generally not constant, and there are demands for a diffraction element having a diffraction efficiency with no polarization dependence. Particularly in the case of a reflection diffraction element, it may be difficult to suppress the polarization dependence, and an element, such as a polarization eliminating element or the like, to compensate for the polarization dependence may be arranged in front of the diffraction element. On the other hand, in the case of the transmission diffraction element, a structure that may suppress the polarization dependence has been reduced to practice.
Examples of the diffraction element are proposed in the International Publication No. 2004/074888 and the Japanese Laid-Open Patent Publication No. 2008-102488, for example.
However, the amount of information transferred in the optical communication is increasing considerably in recent years, and for this reason, the transmission diffraction element that may be used as the diffraction element of the tunable laser and the wavelength selective switch may be required to have a high wavelength resolution, a high diffraction efficiency, and a low polarization dependence throughout the entire wavelength region of the incident light.
In order to obtain a higher wavelength resolution, it may be effective to reduce the period of the gratings of the diffraction element. However, when the period of the grating becomes less than or equal to one-half the wavelength of incident light, in principle the diffraction no longer occurs. Hence, a plurality of diffraction elements may be used, or the light may be transmitted through a single diffraction element a plurality of times, in order to increase the wavelength resolution. In such cases, because the utilization efficiency of light by the diffraction element becomes an exponential of the diffraction efficiency, a higher diffraction efficiency may be required. Hence, the utilization efficiency of light by the diffraction element greatly differs depending on the diffraction efficiency. For example, when the diffraction efficiency is 98% when the light passes through the diffraction element once, the diffraction efficiency becomes approximately 96% when the light passes through the diffraction element two times, and the diffraction efficiency becomes approximately 92% when the light passes through the diffraction element four times. On the other hand, when the diffraction efficiency is 96% when the light passes through the diffraction element once, the diffraction efficiency becomes approximately 92% when the light passes through the diffraction element two times, and the diffraction efficiency becomes approximately 85% when the light passes through the diffraction element four times.
As methods of obtaining a higher diffraction efficiency, the International Publication No. 2004/074888 and the Japanese Laid-Open Patent Publication No. 2008-102488 propose methods of adjusting the structure or cross sectional shape of grating convex parts of the diffraction element. According to these proposals, the grating convex part has a 3-layer structure, and a high diffraction efficiency may be obtained. However, in order to obtain the high diffraction efficiency, the cross sectional shape may need to be a barrel shape or the like, and it may be difficult to form the grating convex part having the barrel-shaped cross section. Another grating convex part that is proposed, having a trapezoidal cross section, may be easier to manufacture. However, the grating convex part having the trapezoidal cross section may have a narrow wavelength range in which a desired diffraction efficiency is obtainable, and a tolerable manufacturing margin at the time of the manufacture may be small. Hence, it may be difficult to obtain a high yield and a high diffraction efficiency throughout the entire wavelength region that is to be used.
In addition, as a method of obtaining a satisfactory wavelength dependence and a high diffraction efficiency the cross sectional in the structure in which the grating convex part has the trapezoidal cross section, the grating convex part may be formed to have the 3-layer structure, and the refractive index (or index of refraction) of each layer in the 3-layer structure and the refractive index of a substrate may satisfy a predetermined relationship. For example, when the refractive indexes of the layers forming the 3-layer structure respectively are n1, n2, and n3 from the layer closest to towards the layer farthest from the substrate, the predetermined relationship may be represented by ns<n1<n2 and n3<n1, where ns denotes the refractive index of the substrate. In this case, because the three (3) layers forming the 3-layer structure are made of mutually different materials, it becomes necessary to use three different kinds of materials to manufacture the 3-layer structure. In addition, a large deposition apparatus may be required to manufacture the 3-layer structure, and the manufacturing process may become complex.
On the other hand, in order to perform signal processing with a satisfactory quality in optical communication, not only a high diffraction efficiency but also reduction of reflected 1st-order diffracted light returning in the same direction as the incident light may be desired. For example, in the case of an optical system in which light emitted from an end of an optical fiber is incident to the diffraction element, when the reflected 1st-order diffracted light is reflected at the end of the optical fiber and again becomes incident to the diffraction element to act as a resonator, the signal intensity may become unstable. For example, when the diffraction efficiency of the transmitted 1st-order diffracted light is 96%, the remaining 4% are distributed to the transmitted 0th-order diffracted light (linearly transmitted light), the reflected 0th-order diffracted light (regular reflected light), and the reflected 1st-order diffracted light (diffracted light returning in the direction of the incident light). In other words, it may be seen that from the point of view of reducing the reflected 1st-order diffracted light, it may be effective to increase the diffraction efficiency of the transmitted 1st-order diffracted light.